1. Field of the Invention
The present invention relates to a radio packet communication system between a master station and slave stations, and more particularly, to a radio packet communication system capable of avoiding transmission collision of packets transmitted by the slave stations.
2. Description of the Related Art
In radio packet communication systems, there is a contention mode and a polling mode.
In the contention mode, a slave station having a transmission request transmits a reservation signal for requesting a transmission to a master station at a voluntary timing, and as a result, the master station gives an exclusive transmission right to the slave station in accordance with the reservation signal. In the contention made, since only slave stations having transmission requests can obtain exclusive transmission right, the transmission can begin immediately without having to wait. However, when the number of slave stations having transmission requests is increased, transmission collision between reservation signals may occur, so that a time required for obtaining the exclusive transmission right may become invalid.
On the other hand, in the polling mode, a transmission right for a reservation signal is sequentially allocated from the master station to all the slave stations. Therefore, there is no collision between transmission data from the slave stations. However, since a transmission right for a reservation signal is allocated to slave stations having no transmission requests, a wait time may be required until slave stations having transmission requests obtain transmission rights, and also, an invalid time period for data transmission may occur.
In a prior art radio packet communication system, in order to effectively make use of both of the contention mode having no wait time and the polling mode having no collision, the contention mode is switched to the polling mode or vice versa in accordance with the occurrence of transmission collision between the slave stations (see JP-A-5-48610).
In the above-mentioned radio packet communication system, however, switching between the contention mode and the polling mode is not smooth.